Control means for industrial heattreating furnaces



vMilly 5 l942- J. G. HooP 2,282,226

CONTROL MEANS FOR INDUSTRIAL HEAT-TREATING FURNACES Filed Sept. '9, 1941gf/0 e /4 an WITNESSES: INVENTOR 0 7707@ Y Jbn /oop.

ATTORNEY r Patented May 5, 1942 CONTROL MEANS FOB INDUSTRIAL HEAT-TBEATING FURNACES f John G. Hooi, Pittsburgh, Pa., signor o Manuflnghoule to Welt- Company,

East Pittsburgh, Pa., a corporation of Penn- Sylvania AppueeuenseptemberI s, 1941, seem No. 410,132

(cris-c4) Claims.

My invention relates, generally, to heat-treating furnaces, and morespecifically relates to heat-treating furnaces in which a, charge is tobe heat-treated by a process which involves bringing the charge from arelatively lower tcmperature to a relatively higher temperature at acontrolled rate of increase in temperature, the charge being envelopedduring the heat-treating process by a gas which is either inert to thecharge, or will react therewith in a desired manner or manners.

in many types oi' heat-treating furnaces, and particularly in furnaceswhich have large heat treating chambers, the heat-treatment is carriadout while the heat-treating chamber is permeated with a controlledatmosphere. Buch a controlled atmosphere may have a relatively high costoi' production, and it is desirable, therefore, to conserve its useduring the heating and cooling of the charge. My invention has this asone of its objects.

Electrical furnaces in which a predetermined heat-treating program orcycle is to be carried out, which program may involve, for example, slowheating from a starting temperature, say ambient temperature, to ahigher soaliinr,r temperature, and slow cooling to a lower temperatureat which the charge may be handled, `or is to be further treated,usually have the power supplied to the heating elements of the furnacevaried, under control of a temperature-responsive means, either byinduction regulators or tap changing transformers, or by intermittentenergizatlon and deenerglzation of the heating-resisters, commonly knownas the cn-off method of' heat control. These arrangements are wellknown, but the former is considerably more costly than the latter.However, in gas-permeated furnaces, particularly those using liquidseals between separable furnace parts, the on-off type of heating`control has the disadvantage that the application of the heat causes arapid expansion of the controlled atmosphere in the heat-treatingchamber. increasing the gas-pressure, so that some of the controlledatmosphere must be released to prevent it from being forced through gasleaks in the furnace, or with furnaces using liquid seals to preventblowing liquid out of the liquid seal used between the separable wallstructure parts of the furnace.

It is an object of my invention to provide in a heat-treating furnace; agas-pressure control means enabling the use of the relatively simple andinexpensive on-off type of temperature without the disadvantages of aloss of controlled atmosphere. or contamination thereof, or deastructiveeffects' on the liquid seal, if one be used between separable parts ofthe furnace wall structure.

In accordance with the application of my invention to velectricfurnaces, a pressure-responsive means is incorporated into the controlof l the energy supply to the heating-resistors, which lilpressure-responsive means prevents the application of heating energy tothe heating-resistors when the pressure of the gas, or controlledatmosphere, therein exceeds any preselected value. Thispressure-responsive means is used in conjunction with atemperature-responsive means which controls the heating or cooling rateof the charge within the furnace.

Other objects, innovations, methods, and features ol my invention, inaddition to those briefiy described above, will be apparent from thefollowing description thereof, which is to he taken in connection withthe accompanying drawingv in which the invention is symbolicallyillustrated for the sake of simplicity, clarity of description andillustration.

In the drawing,

Figure l is a schematic cross-sectional view of a heat-treating furnace,having separable outer wall structure parts, to which my invention maybe applied.

Fig. 2 is a graphical view of a heat-treating cycle which can be carriedout in the furnace of Fig. l; and

Fig. 3 is a symbolic wiring diagram of a control means for obtaining aheating cycle in accordance with Fig. 2, while utilizing the features ofmy invention.

The furnace of Fig. 1 is illustrative of any suitable heat-treatingfurnace to which my irlvention may be applied. The furnace illustratedis one having separable top and bottom structures, such as for example,a bell furnace, and is indicated in its entirety by the referencenumeral 2. The bell furnace comprises an outer wall structure comprisinga base part I and a heating bell part 5 which can be removably placed onthe base part I, and cooperates therewith to deflnevan enclosedheat-treating chamber 8.

The furnace may be either round or rectangular or any other desiredshape, but in this instance the furnace is intended to be of rectangulartype. Consequently, the base part 4 comprislng a rectangulargas-impervious metal base plate I0 supported on any suitable frameworkor founcontrol for controlling the heat-treating cycle, datlon, andcarrying heat insulating and refractory brickwork i2 spportinga centralcharge-` supporting hearth I4. The base part 4 is further provided witha peripheral trough means comprising a U-shaped sealing trough i6gastightly secured around the base plate l0. The heating bell part Sicomprises any suitably constructed heat insulating and refractory.bricln work i3 encased by au outer gas-tight metal provided withsuitable eye-bolts, or the-like, for

raising or lowering the bell, as is customary. The seal between theheating bell 5 and the base part it of the furnace is completed by esealing liquid 2e in the trough it.

For heating the heat=treating chamber it and,

consequently, any cherge resting on the heerth it, the furnace isprovided with any suitable teinperature changing means roi changing thetemperature of the heegt=treating chamber, such as; .for example,radiant gets-med tubes or electricel heating-resistors t@ supported onthe heating bell and heving suitable gas=tight terminals on the outsidethereoie 'Controlled atmosphere is red to the heattreatingchamber 8through e gas inlet pipe t@ and may be withdrawn, or vented, through -agas outlet pipe et, the ges inlet end gas outlet pipes passinggas-tightly through the base part il on opposite sides or thechargeusupporting hearth it. i have indicated the gas inlet pipe 28 asconnected, to any suitable controlled etmosphere sourcevor generatingsystem ti, and, ii

l desired. the gas outlet pipe @il may be return-connected to the gessystem to provide for recirculen tion of the gas. The controlledatmosphere, which may be any suitable gas, a Coon ges i'or heat-treatingsilicon sheet steel in a furnace of the type described includingnitrogen, isied to the heat-treating chamber t in order to gesa permeatethe chber and to gas-envelope a charge on the hearth it. The gas outletpipe t@ also provides a convenient means for per mitting a smallquantity of the gas to be constantly withdrawn from the gas system inorder to maintain the atmosphere within the heattreating chamber t cleanand pure; and a gas valve S2 may control the discharge rate of this Atemperature-responsive means 34, comprisn ing dissimilar wires 36 and 38joined at a Junction- 40 within the heating chamber, extends .throughthe heating bell 5, preferably surrounded by a closed lmetal-refratwtowtube 42 gas-tightly secured to the casing 20 to prevent gas-leakage.

A small pipe passes through a wall of the furnace, for example, a wallof the bell, being gas-tightly secured to the bell casing 20. The pipe60 has an open end exposed to the interior of the heat-treating chamber8 and has its other end closed by a pressure-responsive switch-device 52gas-tightly secured thereto.A The switch-device 52 has a contactor 54(see Fig. 3), movably responsive to the gas-pressure in theheat-treating chamber 8. The switch-device 52 has a lowpressure contact5B' and a high-pressure contact 58, the contactor 54 being movablebetween the contact 58, which is engaged by the contacter 54 when thepressure of the gas in the heattreating chamber 8 is below a certainvalue, and the contact 58 which is engaged by the contacter 54 when thepressure of the gas is above a. ceressence tain higher value.ressure-rsponsive switches l of this type are well known, but I preferto use one in which the contactor 54 must move for some adiustabledistance after leaving oneV of its cooperating contacts before itengages the other contact so that there is an adjustable intermediaterange oi gas-pressures, generally indicative of proper operatinggas-pressure conditions in the furnace, during which the contacter te isout' of engagement with either contact.

When a charge is to be brought from a relatively low temperature up to ahigh temperature at e predetermined rate, or vice versa, the change intempereture may change the gas pressure within the heet-treating chamber8 beyond tolereble limits; end if the relatively low-cost on-aofi"system is used for controlling the application or energy'to theheating-resistors 2t, rapid and excessive changes in. gas pressures mayhe caused by the energization and deenergization oi theheatingaresistors 2t. 'Io avoid such undesireble gaspressure conditions,the.

pressure-responsive switch-device t2 is superimposed on any suitableomen control equipment. Such "on-oiim control equipment is common endvarious forms ere known to those versed in the srt., However, in Fig. 3an equivalent structure has been symholicelly shown in a form muchsimpier than the somewhat involved and complicnted control equipment onthe marlret,v

. but which it suitebie for illustrating the function.A oi suchmechanisms. This symbolic arrengement is contained within the rectangleet, and is shown es comprising a pivoted contact pointer d2 having nposition determined by the temperature within the heat-treating chamberu through connection of the wires 3c and tu of `thetempereture-resporsive means t4 to e suit= eblepointer-moving means Se.The pointer t2 is in wiping engagement with a movable arcuate segment(it consisting oi e conducting portion and an insulating portion iii.The segment to is concentric tothe pivot or the contact pointer t2,which is movable in the arc of a circle havthe periods in which it isenergized, the motor the heat-treating chamber 8, the pointer 82 will.

also move clockwise under the action of the temperature-responsive means34, and so long as the movement of the pointer, which is responsive tothe temperature within the heat-treating chamber 8, does not move fasterthan the segment 66. itv willfbe in engagement with th'e conductingportion of the segment 86. In such engaging position, theheating-resistors 28 canl be energized if the gas pressure in thefurnace is not excessive, as will later be described. If,

however, the temperature of the heat-treating chamber 8 should risefaster than the selected desired rate, then the pointer 62 will movefaster than the segment 66, which moves in proportion to the desiredheating rate, so that the pointer will leave the conducting portion 68and contact the insulating portion 10. This will cause the energy supplycircuit to the heating-resistors 26 to be interrupted, therebydeenergizing them, and stopping the supply of heating energy to theheat-treating chamber 8. The clockwise movement of the pointer slowsdown, and mayA even 'stop or reverse if the chamber 8 cools somewhat. Intime, the conducting portion 66 will overtake the pointer 62 and reclosethe circuit between the conducting portion 68 and pointer 62 to causethe reapplication of energy to the heating-resistors 26, and thus it isapparent that the increase in temperature within the heattreatingchamber 8 can take place at a substantially constant rate under controlof the onoif system represented within' the rectangle 60.

Fig. 2 shows a heating program for heattreating a charge. Assuming Athatthe initial temperature of the charge is represented by the origin, andassuming that the charge is to be heated to a soaking temperature T. ina time a, the travel of the segment 66 is adjusted in accordance withthe slope of the line Os, and the timer 82 is adjusted to maintain themotor 16 energized for a time Oa. Whenrthe temperature Ts is reached,representative of the soaking temperature at which the charge is to besoaked for a time ab, the dividing line 90 between the conductingportion 68 and the insulating portion 'l0 of the segment 66 should be ata point representative of the temperature Ts. However, the timer 82 mayAbe made operative for energizing the motor 16 for a time slightlygreater than the time Oa, in which case an adjustable stop 92 may beprovided in the path of the movable segment 6B for stopping movement ofthis segment when the divided line 80 reaches its limiting position, theslip clutch 80 permitting the motor to continue the runfor the slightlyexcess time period. By adjusting the position of the stop 92, thesoaking temperature T5 can be varied, and the adjustable speed reducinggearing 18 can be adjusted to determine the rate at which the charge inthe heat-treating chamresponsive means, including the switch-device 52is superimposed on the control of the heatisrg-resistor by the pointer62 and the segment A heating control circuit 8|, connected across powerconductors |00 and |02, includes a thermal fuse |04, preferably placedin the heat-treating chamber 8 to prevent excessive temperaturestherein, a pair of separated contacts |06 adapted to be bridged by arelay contactor |08 under control of the switch-device 52, a start-stopswitch ||0, the conducting portion 68 of the segment 66, the pointer 62when engaging the conducting portion 68, andza coil ||2 of a relay H4.The relay ||4 controls an auxiliary circuit ||6 including a contactor||8 raised by energization of the coil 2, to bridge a pair of contacts|20 for energizing the coil |22 of a relay |24. The circuit ||6 may alsoinclude a normally closed contactor |26 bridging a pair of contacts |28of an overload device |30 in the heating-resistor energizing circuit.The relay |24 includes a normally open contactor |32 adapted to engage apair of contacts |34 upon energization of the coil |22, to close anobvious circuit to an operating coil |36 of a heavy duty circuit makerand breaker |31 having normally open contacts |38, which, in closedposition, close the energizing circuit |38 from the three-phase powerleads 84. This energizing circuit |38 passes through the overloadprotective device\|30 to the heating-resistors 26, connected in star.

In the operation of the system thus far described and assuming that thestart-stop'switch I0 is closed to starting position, and that thecontactor |08 bridges its associated contacts |06, which representssatisfactory gas-pressures in the furnace, it is evident that if thepointer 62 is in engagement with the conducting portion 68V of thesegment 66, the coil ||2 of the relay H4 will be energized. Energizationof the coil ||2 causes the normally open contactor ||8 to engage itsassociated contacts |20, thereby completing the circuit between thepower conductors ber 8 is to be brought up to the soaking temperature.

During the soaking period, the pointer 62 may oscillate bach and forthacross the dividing line 90, stationary at a position indicative of thescaliing temperature TE. As the chamber 8 cools the pointer movescomiterclocltwise to engage the'conducting segment 68, reapplying theheating energy to raise the temperature of the heating chamber to thevalue Tt, and when the temperature goes beyond this point, the pointeren- |00 and |02, which includes the coil |22 of the relay |24,Energization of the coil I 22 causes the normally open contactor |32 tobridge its associated contacts |34 to connect the operating coil |36across the power conductors |00 and H12, which causes the contactors N8to move to circuit making position or closing the energizing circuit|39, thereby energizing the heating-resistors 26.

The application of heat to the heating chamber 8 causes the temperatureto rise so that the pointer 62 moves clockwise. However, theheating-resistors 26 are usually designed to heat the heat-treatingchamber 8 at a rate faster than the rate at which the charge is to bebrought up to temperature, so that the heat-treating chamber tends toheat up faster than the desired rate.

Consequently, the pointer 62 will move faster gages the insulatingportion i8 and deenergizes the heating units te stop the application ciheat to the chamber d.

For cooling the charge after a time 0b, the timer 82 reenergizes themotor "i6, but causes it to run in a reverse direction by, for example,reversal of two of the three lphase connections to the motor, so thatthe segment 66 now moves counterclockwise` at a rate in accordance withthe rate of which it is desired to cool the charge,l

and the pointer 62 will tend to follow the movement of the segment inaccordance with the principles of operation previously described.

In accordance with my invention, pressurethan the segment 66, intimatelyleaving the ccnductlng portion 68 of the segment 66, and engaging theinsulating portion 'i0 so that the circuit including the operating coil|i2 ci the relay tit is interrupted, causing it to release its`contactor ||8. Release ci the contactor H8 lnterrup the circuit totheoperating coil |22 causing it to release its contactor |32, andv therebyinterrupting the energizing circuit for the operating coil |36.Deenergization of the operating coil |36 cause it to release its circuitmaker contactors |38 thereby deenergizing the heating-resistors 26.

trol of the pointer 62 resistors 28 completed.

'leaves its contact 58 and `.ingresistors can be energized in responseAt the start or me heating cycle. the switch- `device 52 is adjusted sothat the contactor 54 will leave the low-pressure contact 58 when thegas-pressure in the furnace goes above a selected value, and will engageand remain in engagement with the high pressure contact 58 when thegas-pressure is above a selected value somewhat higher than that forwhich the contact 58 is set:

Assuming a heating cycle /is to be carried out,v and that thegas-pressurein the furnace is such that the contactor 54 circuit'iscompleted from the power conductor' to the contact 58, through thecontactor 54..

through the coil |50 oi a relay |52, through a protectivecurrent-limiting resistor |54, to the other power conductor |02.Energization of the coil |50 raises the normally open contactor |08,whereby the heating control circuit 8| is made operable. Energization ofthe coi1.l50 also completes an obviousl holding circuit forthe relay|52, through its own normally open contactor |58. So long as the controlcircuit 8| is placed in operable condition by engagement of thecontactor |08 on its contacts |05, the application of energy to theheating-resistors 28 is under conand the segment 88 ofthe on-offequipment. Under the assumed lconditions, the relay-coils ||2, |22 and|38 will be energized, and the energizing circuit to the heating- If,upon energization of the heating means in the chamber 8, the gaspressure in the chamber 8 should become so'l great that the contactor 54of the pressure-responsive switch-device 52 moves to the right inFig..3, no change will occur immediately since the contactor |08 is heldin closed position against its contacts |08, because the holding circuitfor the coil |50 is completed through thel contactor |58.l When thecontactor 54 reaches the contact 58, indicative of a. predeterminedgas-pressure, vthe coil |50 is shorted throughs.

Vbranch circuit including the contactor 54 and the contact 58, engagedthereby, the resistor |54 limiting the current-flow through the shortingcircuit. This causes the release of the contactors |08 and |58.

Release of the contactor |08 interrupts the heating control circuit 8|,regardless of the position oi the pointer 82, and the relays itcontrols, to effect the deenergization of the heating-resistors 28,which are energizable only when the controlicircuit 8| is made operableby engagement of the contactor |08 and contacts |08. As the gas pressurebegins to decrease, the contactor 54 moves to the left, and when the gaspressure has decreased sufciently so that the contactor 54 engages thecontact 58, the relay |52 is again energized, causing the engagement ofthe contactor |08 with its contacts |08, to render the4 control circuit8| again operable for controlling the energization of theheating-resistors 28. I! the temperature-responsive means 84 is callingfor the application of heat at this'time, and the pointer 82 is inengagement with the conducting portion 88 of the segment 88, theheating-resistors will be energized. If, however, the temperature of theheat-treating chamber is still above that which is called for by theline of Fig. 2, the

heating-resistors will not'be energized because the pointer 82 will bein engagement with the insulating portion 1 0 oithe segment 88 at thetime.

From the foregoing, it is evident that the heatthe thermo-responsivemeans only while the gaspressure in the heat-treating chamber 8 is belowa set value, which is determined by the setting cf the switch-device t2.

. '5 8 and 58, and also for adjusting the distancefbeis on its contact58, a

tween them to prevent continuous iiutterlng contact oi? the contacter|08 on its contacts |08. The holding circuit for the relay |52, whichincludes the contactor |58, maintains the contactor |08 in closedposition against its contacts |08 when the contacter 54 is moved from aposition against the contact 58 to a position against the contact 58. Inthe reverse movement of the contactor 54, the circuit to the relayA |52is kept open until the contact 58 is engaged by the contactor 54.Consequently, there can be no hunting, that is, rapid intermittentenergization and4 deenergization of the heating means when the pressureof the gas in the heat-treating chamber '8 is such that the contacto'r`54 is just about t0 -A matically shown-in Fig. 3, may be varied con--leave either the contact 58 or the contact 58.

If a multi-zone heating control is used in a furnace, a` singlepressure-responsive means may be used to control all of the individualzone controls, a contactor |08' actuated by coil |50 and l connected ina system such as that embracing contactor |08, being provided for eachzone control.l

While I have shown my invention in a symbolicV form of elements, it isevident that many modifications and equivalents may be used.Theprticular details of the pressure-responsive device or thetemperature-responsive means, or both, may take various forms known tothe art, and the apparatus and connections which are schesiderably.

I claim as my invention: l. A heat-treating furnace comprising, incombination, a wall structure defining a substantially enclosedheat-treating chamber, variably-operable temperature-changing means forsaid heat-treating 'cham-ber, temperature-responsive means responsive,in some manner, to the temperature in said heat-treating chamber,pressure-responsive means responsive, in some manner, to gas-pressure insaid heat-treating chamber, and means 'controlled jointly by saidtemperature-responsive-'means and said pressure-responsive means, forcontrolling the variable operation of said temperature-changing means.

2. The combination with a heat-treating iurnace comprising an outer wallstructure delining a substantiallyenclosed heat-treating chamber,gas-inlet and gas-outlet means for said heat-treating chamber forpermeating said lchamber with a controlled atmosphere, heating means forheating said heat-treating chamber, and control means, comprisingtemperature-responsive means, for variably energizing said heating meansfor selectively controlling the temperature rise oi said heat-treatingchamber y during a heating-up part of a heat-treating cycle: of meansresponsive in some manner, to gas-pressure in said heat-treatingchamber, the las; said means causing said control means to decrease theenergization of said heating means when the gas-pressure is above acertain value.

3. A deviceof the class described, comprising an outer wall'structure ofseparable wall parts, the wall parts cooperating, when together, to denea substantially enclosed heat-treating chambei', said wall parts havingouter metal casing means. and cooperable sealing means, containingliquid, for gas-tightly sealing said heat-treating chamber, heatingmeans carried by said wall structure for heating said heating chamber;means for permeating saidheatlng chamber with a controlled atmosphere;control means, including temperature-responsive means responsive to thetemperature of said heat-treating chamber, for controlling theapplication oi' energy to said heating means whereby the heating cyclein said heat-treating chamber over a period of time is controlled; saidcontrol means, when operable, energizing and deenergizing said heatingmeans in accordance with the temperature respome of saidtemperature-responsive means; and means responsive, in come manner, tothe gas-pressure in said heat-treating chamber for controlling theenergizing operations of said control means on said heating means.

t. A heat-treating heli-type furnace comprising, in combination, afurnace base provided with a peripheral trough means adapted to hold aliquid, a removabie heating bell for said base, cooperating therewith toprovide a heat-treating chamber, said heating bell having an outer metalcasing providedl with a peripheral fiange adapted to be disposed in saidtrough means to provide a sealing means for gas-tightly sealing saidchamber, electric heating means in said Vfurnace for heating saidheating chamber, control means operable Afor energizing and deenergizingsaid electrical heating means for controlling the temperature conditionsof said heating chamber, and means responsive, in some manner, to thegaspressure in said heating chamber, for controlling the operations ofsaid control means for permitheating ting heating of said heatingchamber by said means only when the gas-pressure is below a certainvalue.

5. A heat-treating furnace comprising a wall structure defining asubstantially enclosed heattreating chamber, means for supporting acharge in said chamber, means for permeating saidheattreating chamberwith a controlled atmosphere, heating means for said heat-treatingchamber, heating-control means for energizing and deenergizing saidheating means, thereby varying the gas-pressure in said heat-treatingchamber, and means responsive, in some manner, to gas-pressure in saidheat-treating chamber, for controiling said heating-control means.

6. A heat-treating furnace comprising a divisible wall .structuredefining a substantially enclosed heat-treating chamber, liquid-sealingmeans between divisions of said wall structure,

means for permeating said heat-treating chamber with a controlledatmosphere, energizable and deen'ergizable heating means for saidheating chamber, heating control means, responsive to temperatures insaid heating chamber, for controlling the energization anddeenergization of said heating means so that the temperature in saidheat-treating chamber follows a Drearranged cycle including graduatedtemperature changes, and pressure-responsive means, responsive to thegas-pressure in said heating chamber, for controlling said heatingcontrol means, whereby the heating control means controls the heatingmeans so that the gas-pressure within said heating chamber tends toremain below a preset limit.

. JOHN G. HOOP.

